The present invention relates to a power distribution system, and more particularly, to a circuit breaker for triggering operation of the power distribution system when at least one of an arc fault, a ground fault or an overload is detected in the system.
Low voltage networks, typically 600 volts and below, are used to distribute electric power in a specified area, such as part of a city, an industrial or a commercial area. Often, the cables in such networks are located underground. Typically, the network is designed to feed at more than one point, and therefore, has multiple sources. Occasionally, the cables fail due to various causes such as thermal degradation, age, moisture or rodent damage. The networks are protected by circuit breakers. However, in order to isolate the faulty cable and therefore to minimize disruption of the networks, cable limiters are provided at the ends of the cables. Cable limiters are fuse-like devices that only react safely to high voltage and low impedance faults, such as those created by phase-to-phase faults.
Wiring (miniature) circuit interrupters and current leakage circuit interrupters are commonly used devices for protecting people and property from fire and dangerous electrical faults. Wiring circuit interrupters are used to protect power lines. First, when excessive current passing through a circuit breaker is converted to heat, the circuit interrupter is tripped by the bending of an internal bimetal. Second, when an electric tool or other metallic object on the load shorts the power line, high current is passed through instantaneously, causing the bimetal to heat up and bend. This causes the electric device to be interrupted by the inner magnet of the circuit interrupter.
It is known in this field that the current leakage circuit interrupter has the ability to detect current leakage that may be present in the power line. It trips the circuit interrupter and so protects people from electric shock resulting from current leakage.
In America, according to current regulations, a ground fault circuit interrupter (GFCI) is presently used in applications where direct human contact is possible. The GFCI, which is able to detect current leakage with high sensitivity, is used in current leakage circuit interrupters. Thus, a GFCI must be installed in all kitchens, bathrooms, parking places, basements or other damp places.
In spite of the wiring circuit interrupter and current leakage circuit interrupter, many electrical fires occur all over the world every year. These are often occurred by an arcing type fault to ground occurs rather than a phase-to-phase fault. Arcing faults typically create root mean square (RMS) current value, which is below the thermal threshold for such circuit breakers. Even so, the arcs can cause damage or start a fire if they occur near combustible material.
Arcs are potentially dangerous due to their high temperatures. An arc, however, will only trip a GFCI if it produces sufficient current leakage to ground. In addition, an arc will trip a circuit breaker only if the current, flowing through the arc, exceeds the trip parameters of the thermal/magnetic mechanism of the circuit breaker. Therefore, an additional type of protection device is needed to detect and interrupt arcs that do not fit these criteria. An arc detector whose output is used to trigger a circuit interrupting mechanism is referred to as an arc fault circuit interrupter (AFCI).
According to the Consumer Product Safety Commission (CPSC), it was estimated that 40% of the fires in 1997 were due to arc faults. The National Electric Code (NEC) requires AFCI installation in all the residential buildings beginning in January 2002. The causes of arcing are numerous. For example, it may be caused by: aged or worn insulation and wiring; mechanical and electrical stress caused by overuse, excessive currents or lightning strikes; loose connection; or excessive mechanical damage to insulation and wires.
Three types of arcing may occur in residential or commercial buildings: series arcing, parallel arcing and ground arcing.
Series (or contact) arcing occurs between two contacts in series with a load. An example of series arcing is illustrated in FIG. 1. The conductors 14, 16 comprising the cable 10, are separated and surrounded by an insulator 12. A portion of the conductor 14 is broken, creating a series gap 18 in the conductor 14. Under certain conditions, arcing will occur across this gap, producing a large amount of localized heat. The heat produced by the arcing might be sufficient to break down and carbonize the insulation 19 close to the point of arcing. If the arc is allowed to continue, enough heat will be generated to start a fire. Under these conditions, current flowing through the arc is controlled by load.
A schematic diagram illustrating an example of parallel (line) arcing is shown in FIG. 2. The cable 20 comprises electrical conductors 24, 26 covered by outer insulation 22 and separated by inner insulation 28. Deterioration or damage to the inner insulation 28 at 21 may cause parallel fault arcing 23 to occur between the two conductors 24, 26. The inner insulation could have been carbonized by an earlier lightning strike to the wiring system, or it could have been cut by some mechanical action such as a metal chair leg cutting into an extension cord.
A schematic diagram illustrating an example of ground arcing occurring between a conductor and the ground is shown in FIG. 3. If the outer insulation 38 for protecting conductors 34, 36 is damaged, the conductor 36 contacting ground at the damaged portion 39 produces ground arcing.
The arcing current may be changed by impedance because parallel arcing and ground arcing occur parallel to the load. The long-term deterioration causes cable carbonization and damage to the coating. The cable is further deteriorated by Joule heat, which is induced by arcing current. The arcing is generated in the following manner: J (Joule heat)=I2 (arcing current)xc3x97t (Time).
An example of static current and arcing current in the resistor load are illustrated in FIG. 4. The arcing current 42 is not normal sine wave but is distorted at the phase changing point. According to the distortion of arcing current, arcing voltage is also distorted. FIG. 5 shows the relation between arcing current and arcing voltage.
An example of distorted AC line voltage caused by arcing current is illustrated in FIG. 6. The Joule heat is increased against the decrease of RMS AC line voltage value 61 caused by irregular arcing current 62. An arc is superposed on the AC line voltage. The frequency of harmonic or overtone is extended to GHz, and it can be seen by spectrum analysis of the frequency of arcing current.
The major problem associated with any type of arc detection is false tripping. False tripping occurs when an arc detector produces a warning output, or disconnects a section of wiring from the voltage source, when a dangerous arcing condition does not actually exist. This problem is caused by the fact that arcing current and arcing voltage are not generated in the form of correct sine wave, and have various types of waveforms. Specifically, arcing current and arcing voltage are similar to the driving pulse created in appliances, such as fans and dryers that have electric motors inside.
FIG. 7 illustrates the signals related to output voltage in the resistor load, and FIG. 8 illustrates the output voltage with arcing. And, FIG. 9 illustrates output voltage waveform in a driving electric device.
The signals in FIG. 7 show that under a normal load, the output voltage is generated to pulse every {fraction (1/60)} sec. The signals in FIG. 8 show that under arcing conditions, arcing voltage with high amplitude is detected every {fraction (1/60)} sec. Also, if you use an electric device, you can see that at the beginning of a cycle, high pulse similar to the arcing voltage is generated, and after a period of time, output voltage will have the normal amplitude (See FIG. 9). Therefore, it is difficult to detect arcing because arcing voltage is similar to a driving pulse at the beginning of a cycle.
The arc fault detector (AFD) in U.S. Pat. No. 5,805,397 discloses the method of detecting arcing by multiple channel sensing. The prior patent uses a method of detecting arcing in several bandwidths, and the AFD trips the circuit under conditions of arcing generation in any bandwidths.
A schematic diagram in block form of this prior art is shown in FIG. 10. The electrical system 100 protected by the circuit breaker 103 includes a line conductor 105 and a neutral conductor 107 connected to provide power to a load 109. The circuit breaker 103 includes separable contacts 111 which can be tripped open by a spring operated trip mechanism 101. The trip mechanism 101 may be activated by a conventional thermal-magnetic over-current device 116. This thermal-magnetic over-current device 116 includes a bimetal 115 connected in series with the line conductor 105. Persistent over-currents heat up the bimetal 115 causing it to bend and release a latch 113, which activates the trip mechanism 101. Alternatively, short circuit currents through the bimetal 115 magnetically attract an armature 114, which releases the latch 113 to activate the trip mechanism 101.
A schematic diagram of a prior art arc fault detection circuit is shown in FIG. 11. The arcing fault detector 120 is a multi-channel bandpass filter circuit 126 including two channels 123, 124. Each channel 123, 124 includes a bandpass filter 125 and 126. Each bandpass filter 125 and 126 has an assigned, distinct non-overlapping passband. Thus each of the bandpass filters 125, and 126 will generate an output signal in response to an arcing fault. Therefore, the circuit breaker is tripped when the accumulated output signal from the filter reaches a specified level.
A block diagram illustrating an arc fault/ground fault circuit interrupter (AFCI/GFCI) device of the prior art is shown in FIG. 12. The prior AFCI generates an output signal comparing the first arc detecting signal in the line with the second arc detecting signal in the load. The AFCI/GFCI device 180 comprises AFCI/GFCI circuitry 182, line circuitry 188, load circuitry 200, arc detection circuitry 198, local/remote inhibit circuitry 184, and timer circuitry 186.
And, FIG. 13 shows a schematic diagram illustrating the AFCI/GFCI circuitry portion of the prior art arc fault detection device in more detail.
The prior art AFCI/GFCI device may control electric circuit processing independently in response to arcing generation, with the result of comparing line arcing and load arcing at each line circuitry 188 and load circuitry 200. However, the prior art needs amplifier, filter, rectifier and peak detector at each line and load circuitry, so it costs more. Furthermore, it is difficult to install an AFCI/GFCI device in a house because of its added size. Also, under the various real-life conditions, the prior arts cannot detect an arcing fault. Therefore, they are not effective for prevention of electrical fires in residential or commercial buildings.
The arc fault circuit interrupter (AFCI) of the present invention can effectively detect arc faults generated in electrical systems, and so protect people and their property from electrical fires.
The AFCI of the present invention can operate in combination with a ground fault circuit interrupter (GFCI) or overload circuit interrupter (OLCI). Thus, the circuit breaker with AFCI, GFCI and OLCI of the present invention can be provided to detect arc faults, ground faults and overloads effectively.
Also, the circuit breaker uses a simple construction and fewer elements. Therefore, it is less expensive and less time-consuming to construct circuit breaker, and can easily be installed in residential and commercial buildings.
To achieve the above-mentioned objects of the present invention, it is provided an arc fault circuit interrupter (AFCI) device in an electrical wiring system that can shut an AC (Alternating Current) source off from a phase conductor and a neutral conductor when an arc fault occurs in the AC source. The arc fault circuit interrupter device may comprise a current transformer for producing an arc voltage in accordance with the variation of current in the phase conductor and in the neutral conductor, an arc fault detector for limiting the arc voltage to a specified level and producing an arc fault indicative signal when the arc voltage is higher than a predetermined level, a trip signal generator for charging the arc fault indicative signal, and if the charged arc fault indicative signal corresponds to a reference trip level, then producing a trip signal, and a trip circuitry coupled between the phase conductor and the neutral conductor, for shutting the AC source off from the phase conductor and the neutral conductor in response to the trip signal.
The arc fault detector may comprise a filter for diminishing a harmonic frequency (i.e., overtone) component from the arc voltage and limiting the arc voltage to a specified level and a comparator for comparing the limited arc voltage with a reference arc signal and producing the arc fault indicative signal based upon the result of comparison. The filter may comprise a level limiter for limiting the arc voltage to the specified level and a rectifier for half or full wave rectifying the limited arc voltage. The level limiter may comprise at least one resistor coupled to the current transformer. The rectifier may comprise a first plurality of diodes coupled between the resistor of said level limiter and ground and a second plurality of diodes coupled between the resistor of said level limiter and said comparator. The comparator may comprise at least one operational amplifier. The comparator may further comprise a reference arc signal generator.
The trip signal generator may comprise a voltage divider for dividing the arc fault indicative signal, a comparator for comparing the divided arc fault indicative signal with the reference trip level and providing a first state output signal when the divided arc fault indicative signal is higher than the reference trip level, a first switch for controlling electrical power being provided to the comparator and a trip level controller for providing the trip signal according to the first state output signal.
The first switch may comprise a common emitter amplifier. The trip level controller may comprise a resistor coupled to output terminal of said comparator and a capacitor in parallel with the resistor. The resistor may be a variable resistor. The trip signal generator may further comprise a second switch coupled between said comparator and said trip level controller, for transferring the first state output signal to said trip level controller. The second switch may comprise an emitter follower.
Also, to achieve the above-mentioned objects of the present invention, it is provided a circuit breaker device in an electrical wiring system that can shut an AC (Alternating Current) source off from a phase conductor and a neutral conductor when at least one of an arc fault, ground fault or overload occurs in the AC source.
The circuit breaker may comprises an arc fault circuit interrupter (AFCI) coupled to the phase conductor and the neutral conductor for detecting an arc fault and producing an arc fault trip signal, a ground fault circuit interrupter (GFCI) coupled to the phase conductor and the neutral conductor for detecting ground fault and producing a ground fault trip signal, an overload circuit interrupter (OLCI) coupled to the phase conductor and the neutral conductor for detecting an overload and producing an overload trip signal, a display circuitry for indicating the arc fault, ground fault or overload respectively corresponding with at least one selected from the group consisting of the arc fault trip signal, the ground fault trip signal and the overload trip signal and a trip circuitry coupled to the phase conductor and the neutral conductor, for shutting the AC source off from the phase conductor and the neutral conductor corresponding with at least one selected from the group consisting of the arc fault trip signal, the ground fault trip signal and the overload trip signal. Wherein the arc fault circuit interrupter may comprise a current transformer for producing an arc voltage in accordance with variation of current in the phase conductor and the neutral conductor, an arc fault detector for limiting the arc voltage to a specified level and producing an arc fault indicative signal when the arc voltage is higher than a predetermined level and a trip signal generator for charging the arc fault indicative signal, and if the charged arc fault indicative signal corresponds to a reference trip level, then producing the arc fault trip signal.
The ground fault circuit interrupter (GFCI) may comprise a current detector for detecting current variation in the phase conductor and in the neutral conductor, and converting the current variation to a ground fault voltage, a filter for limiting the ground fault voltage to the specified range, a comparator for comparing the range limited ground fault voltage with a reference voltage, and producing a ground fault indicative signal in accordance with the result of comparison and a delay circuitry for producing the ground fault trip signal when the ground fault indicative signal is not less than a ground fault trip level. The filter further may comprise a power supply for diminishing noise from power and providing noise-free power. The delay circuitry may comprise a resistor and a capacitor in parallel with the resistor.
The overload circuit interrupter may comprise an overload detector coupled to the phase conductor and the neutral conductor for detecting the overload and producing an overload indicative signal and an overload trip signal generator for providing the overload trip signal in accordance with the overload indicative signal when the overload occurs. The overload detector may comprise at least one bimetal. The overload trip signal generator comprises metal oxide varistor.
The trip circuitry may comprise a power interrupter for shutting the AC source off from the phase conductor and the neutral conductor if an arc fault, the ground fault or overload occurs and a trip controller for controlling said power interrupter corresponding with at least one selected from the group consisting of the arc fault trip signal, the ground fault trip signal and the overload trip signal. The power interrupter may comprise a pulse generator for providing a magnetic pulse as a result of a fault in a current and a switch circuit for interrupting the AC source by means of the magnetic pulse. The pulse generator may be a solenoid. The trip controller may comprise a silicon controlled rectifier (SCR) and at least one diode for directing path of the current flow in said trip circuitry in accordance with generation of the magnetic pulse. The trip controller further may comprise a pressure controller. The pressure controller may comprise a first node coupled to the at least one diode of the trip controller and the varistor, a second node coupled to the phase wire and a third node coupled to the first node, wherein, the first node is connected to the second node or third node in accordance with an operation of said switch circuit.
The display circuitry may comprise a plurality of display elements for indicating at least one selected from the group consisting of the arc fault, the ground fault and the overload, a display controller for controlling the operation of said display elements and a power supply for providing power to said display elements. The display elements may comprise at least one light emitting diode. The display controller may comprise a transistor connected between both ends of said display elements.